Laser Despeckler Based on Angular Diversity

ABSTRACT

A device for reducing laser speckle using a micro scanner and a holographic diffuser. The micro scanner includes a first transparent optical substrate with an input surface and an output surface and a second transparent optical substrate with an input surface and an output surface and a variable refractive index medium sandwiched between the output surface of the first substrate and the input surface of the second substrate. Transparent electrodes are applied to the output surface of the first substrate and the input surface of the second substrate. The electrodes are coupled to a voltage generator. The input surface of the first substrate is optically coupled to a laser source. The input surface of the second substrate is configured as an array of prismatic elements. At least one of the input surface of the first substrate or the output surfaces of the second substrate is planar.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/043,806 filed Feb. 15, 2016, which application is a continuation ofU.S. application Ser. No. 14/009,637 filed Oct. 8, 2013, whichapplication is the U.S. national phase of PCT Application No.PCT/GB2012/000331 filed Apr. 10, 2012, which application claims thebenefit of U.S. Provisional Application No. 61/457,482 filed Apr. 7,2011, the disclosures of which are incorporated herein by reference intheir entireties.

BACKGROUND

The present invention relates to an illumination device, and moreparticularly to a laser illumination device based on electricallyswitchable Bragg gratings that reduces laser speckle.

Miniature solid-state lasers are currently being considered for a rangeof display applications. The competitive advantage of lasers in displayapplications results from increased lifetime, lower cost, higherbrightness and improved color gamut. Laser displays suffer from speckle,a sparkly or granular structure seen in uniformly illuminated roughsurfaces. Speckle arises from the high spatial and temporal coherence oflasers. Speckle reduces image sharpness and is distracting to theviewer.

Several approaches for reducing speckle contrast have been proposedbased on spatial and temporal decorrelation of speckle patterns. Moreprecisely, speckle reduction is based on averaging multiple sets ofspeckle patterns from a speckle surface resolution cell with theaveraging taking place over the human eye integration time. Speckle maybe characterized by the parameter speckle contrast which is defined asthe ratio of the standard deviation of the speckle intensity to the meanspeckle intensity. Temporally varying the phase pattern faster than theeye temporal resolution destroys the light spatial coherence, therebyreducing the speckle contrast. Traditionally, the simplest way to reducespeckle has been to use a rotating diffuser to direct incident lightinto randomly distributed ray directions. The effect is to produce amultiplicity of speckle patterns while maintaining a uniform atime-averaged intensity profile. This type of approach is often referredto as angle diversity. Another approach known as polarization diversityrelies on averaging phase shifted speckle patterns. In practice neitherapproach succeeds in eliminating speckle entirely.

It is known that speckle may be reduced by using an electro optic deviceto generate variations in the refractive index profile of material suchthat the phase fronts of light incident on the device are modulated inphase and or amplitude. The published International Patent ApplicationNo. WO/2007/015141 entitled LASER ILLUMINATOR discloses a despecklerbased on a new type of electro optical device known as an electricallySwitchable Bragg Grating (SBG). An (SBG) is formed by recording a volumephase grating, or hologram, in a polymer dispersed liquid crystal (PDLC)mixture. Typically, SBG devices are fabricated by first placing a thinfilm of a mixture of photopolymerizable monomers and liquid crystalmaterial between parallel glass plates. Techniques for making andfilling glass cells are well known in the liquid crystal displayindustry. One or both glass plates support electrodes, typicallytransparent indium tin oxide films, for applying an electric fieldacross the PDLC layer.

A volume phase grating is then recorded by illuminating the liquidmaterial with two mutually coherent laser beams, which interfere to formthe desired grating structure. During the recording process, themonomers polymerize and the HPDLC mixture undergoes a phase separation,creating regions densely populated by liquid crystal micro-droplets,interspersed with regions of clear polymer. The alternating liquidcrystal-rich and liquid crystal-depleted regions form the fringe planesof the grating. The resulting volume phase grating can exhibit very highdiffraction efficiency, which may be controlled by the magnitude of theelectric field applied across the PDLC layer. When an electric field isapplied to the hologram via transparent electrodes, the naturalorientation of the LC droplets is changed causing the refractive indexmodulation of the fringes to reduce and the hologram diffractionefficiency to drop to very low levels. Note that the diffractionefficiency of the device can be adjusted, by means of the appliedvoltage, over a continuous range from near 100% efficiency with novoltage applied to essentially zero efficiency with a sufficiently highvoltage applied. U.S. Pat. No. 5,942,157 and U.S. Pat. No. 5,751, 452describe monomer and liquid crystal material combinations suitable forfabricating SBG devices. An SBG device typically comprises at least oneSBG element that has a diffracting state and a non-diffracting state.Typically, the SBG element is configured with its cell wallsperpendicular to an optical axis. An SBG element diffracts incidentoff-axis light in a direction substantially parallel to the optical axiswhen in said active state. However, each SBG element is substantiallytransparent to said light when in said inactive state. An SBG elementcan be designed to diffract at least one wavelength of red, green orblue light. SBGs may be stacked to provide independently switchablelayers.

SBGs with Bragg grating pitches much smaller than the operatingwavelength exhibit form birefringence in other words they behave like anegative uniaxial crystal with an optic axis perpendicular to the Braggplanes. They are referred to as sub-wavelength gratings. The incidentwave cannot resolve the sub-wavelength structures and sees only thespatial average of its material properties. Only zero order forward andbackward “diffracted” waves propagate and all higher diffracted ordersare evanescent. The birefringence is switched off when the refractiveindices of the PDLC and polymer planes are equal. The retardance of asub wavelength grating is defined as the difference between theextraordinary and ordinary refractive indices multiplied by the gratingthickness. As will be discussed later subwavelength gratings can be usedto provide a variable refractive index medium.

There are two types of speckle known as objective speckle and subjectivespeckle. Objective speckle occurs as a two dimensional random pattern ona projection screen and has the effect of degrading the resolution ofthe projected image. Subjective speckle manifests itself as floatinglight spots that the eye cannot focus on. It does not affect the imageon the screen surface. Classical methods for overcoming speckle rely onthe principle of randomly displacing a diffusing surface relative to thelaser illumination beam. The relative displacement is usually providedby a rotating diffusing screen. Another equivalent solution is to have astatic diffusing screen and a means for scanning the laser illuminationacross the screen. However, such approaches have failed to deliver thelevels of speckle contrast reduction required by modern laser displaytechnology. Mechanical scanning solutions also suffer from the problemsof mechanical and optical design complexity, noise and cost ofimplementation. There is a need for a compact solid state solution tothe problem of speckle reduction using the principle of angulardiversity.

There is a requirement for a despeckler with improved speckle contrastreduction.

SUMMARY

It is a first object of the present invention to provide a despecklerwith improved speckle contrast reduction.

In one embodiment of the invention there is provided a device forreducing laser speckle comprising: a micro scanner and a holographicdiffuser.

The micro scanner device comprises: a first transparent opticalsubstrate with an input surface and an output surface; a secondtransparent optical substrate with an input surface and an outputsurface and a variable refractive index medium sandwiched between theoutput surface of the first substrate and the input surface of thesecond substrate. Transparent electrodes are applied to the outputsurface of the first substrate and the input surface of the secondsubstrate. The electrodes are coupled to a voltage generator. The inputsurface of the first substrate is optically coupled to a laser source.The input surface of the second substrate is configured as an array ofprismatic elements containing surfaces. Advantageously, at least one ofthe input surface of the first substrate or the output surfaces of thesecond substrate is planar.

At least one of said transparent electrodes is patterned intoindependently addressable electrode elements. The average refractiveindex of any region of said variable refractive index medium isproportional to the voltage applied across the electrode elementssandwiching said region. The micro scanner deflects input light from thelaser source into output light at an angle determined by the refractiveindex of the substrates and the average refractive index of the variablerefractive index medium. The voltage applied across each electrodeelement is varied temporally. Each point in the holographic diffuserdiffracts incident light rays of a predefined angle into output lightrays having a predefined range of angles to form a diffuse illuminationpatch.

In one embodiment of the invention a despeckler according to theprinciples of the invention comprises a micro scanner and a holographicdiffuser. The micro scanner is illuminated by light from a laser whichis expanded and collimated by the beam coupling optics. The microscanner deflects the beam in small angular sweeps of random amplitude.The holographic diffuser then diffracts light to form a diffuseillumination patch.

In one embodiment of the invention the despeckler further comprises abeam steering means for directing the output ray angles from themicro-scanner into the input angles required by the holographicdiffuser. Advantageously the beam steering means is a diffractive devicebased on Bragg gratings.

In one embodiment of the invention a projector incorporating thedespeckler further comprises a microdisplay, a projection lens, and ascreen, which is observed from an eye position.

In one embodiment of the invention at least two micro scanners ofidentical prescriptions are provided. The micro scanners are stacked andoperated independently.

In one embodiment of the invention the beam coupling optics comprises aTIR lightguide. A coupling grating admits collimated light from thelaser into a TIR path. A second coupling grating directs light into themicro scanner. A TIR Iightguide may be used to couple in light from RGBlaser sources or multiple monochromatic sources.

In one embodiment of the invention the holographic diffuser is aComputer Generated Hologram (CGH).

In one embodiment of the invention the holographic diffuser is recordedinto a Holographic Polymer Dispersed Liquid Crystal (HPDLC).

In one embodiment of the invention the holographic diffuser also encodesthe properties of beam shaping and homogenization.

In one embodiment of the invention illustrated in there is provided adespeckler for used with a reflective display which further comprises apolarizing beamsplitter and a quarter wave plate. Linearly polarizedlight from the laser is transmitted through the micro scanner, beamcoupling optics and holographic diffuser illumination light which istransmitted through the polarizing beamsplitter transmitted through thequarter wave plate reflected at the microdisplay and transmitted onceagain through the quartet wave plate emerging with polarizationorthogonal to that of the incident light and is then reflected at thepolarizing beam splitter towards a projection lens. In the case of aliquid crystal display panel the quarter wave plate will not berequired.

In one embodiment of the invention the micro scanner is polarizationsensitive.

In one embodiment of the invention both of the transparent electrodes inthe micro scanner are continuous. The variable index material isselectively switched in discrete steps from a fully diffracting to a nondiffracting state by an electric field applied across the transparentelectrodes.

In one embodiment of the invention at least one of the transparentelectrodes in the micro scanner is patterned to provide independentlyswitchable electrode elements such that portions of the variable indexmaterial may be selectively switched from a diffracting to a nondiffracting state by an electric field applied across the transparentelectrodes. Desirably, the electrodes are fabricated from ITO.

In one embodiment of the invention the electrode elements in the microscanner have substantially the same cross sectional area as a prismaticelement.

In one embodiment of the invention the center of said electrode elementin the micro scanner overlaps the vertex of a prismatic element.

In one embodiment of the invention the center of an electrode element inthe micro scanner is offset from the vertex of a prismatic element.

In one embodiment of the invention the prism array in the micro scanneris a linear array of elements of triangular cross section.

In one embodiment of the invention the prism array in the micro scanneris a two-dimensional array comprising pyramidal elements.

In one embodiment of the invention the prismatic elements in the microscanner are identical.

In one embodiment of the invention the surface angles of the prismaticelements in the micro scanner have a random distribution.

In one embodiment of the invention the prismatic elements in the microscanner are each characterized by one of at least two different surfacegeometries.

In one embodiment of the invention the prismatic elements in the microscanner are each characterized by one of at least two different surfacegeometries with the prismatic elements of a given surface geometry beingdistributed uniformly across the prism array.

In one embodiment of the invention the prismatic elements in the microscanner have diffusing surfaces.

In one embodiment of the invention the variable refractive index mediumis a subwavelength grating.

In one embodiment of the invention the variable refractive index mediumis a HPDLC material.

In one embodiment of the invention the variable refractive index mediumis a SBG.

In one embodiment of the invention the laser source comprises red greenand blue emitters.

In one embodiment of the invention the micro scanner further comprises abeam shaping diffuser.

In one embodiment of the invention the micro scanner further comprises abeam collimating lens.

In one embodiment of the invention the micro scanner further comprises abeam shaping diffuser and at least one beam collimating lens.

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings wherein like index numerals indicate like parts.For purposes of clarity details relating to technical material that isknown in the technical fields related to the invention have not beendescribed in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevation view of a micro scanner.

FIG. 2 is a schematic side elevation view of one aspect of a microscanner.

FIG. 3A is a schematic side elevation view of a first aspect of a microscanner.

FIG. 3B is a schematic side elevation view of a second aspect of a microscanner.

FIG. 3C is a schematic side elevation view of a third aspect of a microscanner.

FIG. 4A is a schematic view illustrating a method of recording aholographic diffuser.

FIG. 4B is a schematic view of a first aspect of the operation of aholographic diffuser.

FIG. 4C is a schematic view of a second aspect of the operation of aholographic diffuser.

FIG. 5 is schematic side elevation view of a despeckler according to oneembodiment of the invention.

FIG. 6A is detail of the operation of a despeckler according to oneembodiment of the invention.

FIG. 6B is detail of the operation of a despeckler according to oneembodiment of the invention.

FIG. 7 is a schematic side elevation view of despeckler according to oneembodiment of the invention.

FIG. 8 is a schematic side elevation view of a projection displayincorporating a despeckler according to one embodiment of the invention.

FIG. 9 is a schematic side elevation view of a projection displayincorporating a despeckler according to one embodiment of the invention.

FIG. 10 is a schematic side elevation view of a projection displayincorporating a despeckler according to one embodiment of the invention.

FIG. 11 is a schematic side elevation view of a projection displayincorporating a despeckler according to one embodiment of the invention.

FIG. 12 is a schematic side elevation view of despeckler for use with areflective microdisplay according to one embodiment of the invention.

FIG. 13 is a schematic side elevation view of despeckler for use with areflective microdisplay according to one embodiment of the invention.

DETAILED DESCRIPTION

It an object of the present invention to provide a despeckler withimproved speckle contrast reduction.

It will be apparent to those skilled in the art that the presentinvention may be practiced with only some or all aspects of the presentinvention as disclosed in the following description. For the purposes ofexplaining the invention well-known features of laser technology andlaser displays have been omitted or simplified in order not to obscurethe basic principles of the invention.

Parts of the following description will be presented using terminologycommonly employed by those skilled in the art of optics and laserdisplays in particular.

In the following description the terms light, ray, beam and directionwill used interchangeably and in association with each other to indicatethe propagation of light along rectilinear trajectories.

Unless otherwise stated the term optical axis in relation to a ray orbeam direction refers to propagation parallel to an axis normal to thesurfaces of the optical components described in relation to theembodiments of the invention.

It should also be noted that in the following description of theinvention repeated usage of the phrase “in one embodiment” does notnecessarily refer to the same embodiment.

The despeckler embodiments disclosed herein are directed at overcomingboth objective and subjective speckle. The devices generate set ofunique speckle patterns within an eye resolution cell by operating onthe angular characteristic of rays propagating through the despeckleraccording to the angular diversity principle.

Specifically the invention provides a despeckler that combines a meansfor generating illumination light with a temporally varying randomdistribution of ray directions to be referred to as a micro scanner witha holographic means for generating a diffuse illumination patch at aspecified location. Said holographic means will be referred to as aholographic diffuser in the following description of the invention. Thelocation of the diffuse illumination patch typically coincides with thesurface of a microdisplay but in some display applications it may beadvantageous for it located at or near to an intermediated image plane.

The invention provides a solid state analogue of the classicalmoving-screen speckle reduction technique. The holographic means forcreating a diffuse illumination patch also provides a means for shapingthe beam cross section and controlling the spatial homogeneity of theillumination all of said features being encoded within a singlehologram.

We start by considering the micro scanner. The basic principles of amicro scanner for use with the invention are illustrated in theschematic side elevation view of FIG. 1. The apparatus comprises: afirst transparent optical substrate 91 with an input surface 91A and anoutput surface 91B and a second transparent optical substrate 93 with aninput surface 93A and an output surface 93B. The input surface of thesecond substrate 93 is configured as an array of prismatic elements eachprismatic element containing surfaces such as 93A. Advantageously, atleast one of the input surface of the first substrate or the outputsurfaces of the second substrate is planar. Transparent electrodes98A,98B are applied to the output surface 91B of said first substrateand the input surface 93A of said second substrate. A variablerefractive index layer 92 having input surfaces 92A, 92B is sandwichedbetween the output surface of the first substrate and the output surfaceof the second substrate providing an array of variable refractive indexprismatic elements. Advantageously the variable refractive index layeris a HPDLC material. In one embodiment of the invention the variablerefractive index layer is a SBG. In one embodiment of the invention thevariable refractive index layer is a sub wavelength grating. The insetsmarked by dashed lies show portions of the firstsubstrate-electrode-HPDLC layer interface and the secondsubstrate-electrode-HPDLC layer interface in more detail. The electrodesare coupled to a voltage generator 90 by means of an electrical circuit99. The input surface 91A of the first substrate 91 is optically coupledto a laser source which is not illustrated. The substrates arefabricated from an optical glass such as BK7. Alternatively, opticalplastics may be used.

We consider the propagation of light through one of the prismaticelements. Input laser light indicated by the rays 440A,440B istransmitted through substrate 91 into the HPDLC. Refracted rays from afirst prism surface 93B are indicated by 441A and refracted rays from asecond prism surface are indicated by 441B. Each of the refracted raysin the groups indicated by 441A,441B corresponds to a unique averagerefractive index resulting from a unique applied voltage. The rays441A,441B are refracted at the output surface of the second substrate 93to provide the output rays 442A,442B. As indicated in the drawing eachprism will provide overlapping rays indicated by the divergent raybundles 440,450,460,470.

The ray geometry is illustrated in more detail in FIG. 2 which providesa schematic illustration of the ray propagation around one prism face.The angle of deflection in the prism is given by α₂=arcsin((n_(h)/n_(g)) sin (α₁), which is approximately equal to (n_(h)/n_(g))α₁. The prism angle α₁ is given by α₁=arctan (h/D), where D is thelength of the prism (or period) and h is its height. It can be shownthat the resulting angle of prism deflection δ is given by δ=arcsin(n_(g) sin(α₂−α₁). Making the approximation that δ=n_(g) (a₂−α₁), weobtain: δ=n_(g) a₂ (n_(h)/n_(g)−1). Combining both previous equations,the deflection angle may be expressed as a function of the prismcharacteristics and index. Based on the above equations the raydeflection is given by δ=n_(g) ((h/D) (n_(h)/n_(g)−1). The directions ofthe output rays are swept by increasing the effective refractive indexin the HPDLC between the substrate-HPDLC index match condition and thefull effective index shift. Typically, the index of glass is n_(g)=1.55.The index of the HPDLC n_(h) in its non diffracting state is matched tothe index of the substrate glass which is typically 1.55. The inventorshave found that the maximum refractive variation of the HPDLC istypically +0.065. The HPDLC material has a sinusoidal sub-wavelengthgrating with a duty cycle of 50% of the index swing regions (brightfringes). Therefore the maximum effective refractive index changeextends from 1.55 to 1.55+0.065/2=1.5825. Assuming a prism height of 1micron, a prism length of 30 microns, and n_(g)=1.55 and n_(h)=1.5825,we obtain a deflection angle of 0.062 degrees.

FIG. 3 illustrates the sweeping of output rays as the voltage appliedacross the HPDLC via the electrodes 98A,98B is varied. At the maximumvoltage condition illustrated in FIG. 3A there is nor deflection in theincoming rays 430 which propagate into the from the HPDLC region 92 intoregion 93 as the rays 431 and subsequently into air as rays 432 withoutdeviation. FIGS. 3B-3C show how the ray deviation increases as thevoltage is reduced. In FIG. 3B input collimated light 433 is deflectedinto the ray directions 434 in the HPDLC medium and into ray direction435 in air. In FIG. 3C input collimated light 436 is deflected into theray directions 437 in the HPDLC medium and into ray direction 438 inair.

In one embodiment of the invention both of the transparent electrodesare continuous. The HPDLC is selectively switched in discrete steps froma fully diffracting to a non diffracting state by an electric fieldapplied across the transparent electrodes.

At least one of said transparent electrodes is patterned to provideindependently switchable electrode elements such that portions of theHPDLC may be selectively switched in discrete steps from a fullydiffracting to a non diffracting state by an electric field appliedacross the transparent electrodes. Desirably, the electrodes arefabricated from ITO.

We next consider the principles of and method of recording of theholographic diffuser referring to FIGS. 4A-4C. The beam shaping deviceis essentially a hologram of a diffuser or scatter plate. Using atraditional holographic recording procedure, which is illustratedschematically in FIG. 4A, the holographic diffuser is recorded byilluminating a holographic recording medium 2A by light scattered from areal diffuser 5 and a second collimated reference beam indicated by therays 201,202. Advantageously, the rays 201,202 are parallel. Groups ofrays from two points on the diffuser are generally indicated by 211 and212. Each group of rays from a point on the diffuser surface fills theaperture of the hologram. Under playback the processed hologramindicated by 2B is illuminated by a beam parallel to the reference beamsuch that the hologram forms a static image of the diffuser. Every pointof the diffuse illumination patch reproduces each divergent point of thediffuser. Two equivalent interpretations may be used to characterize theformation of a diffuse illumination patch by the hologram. Referring toFIG. 4A it will be seen that each point on the hologram diffractsincident light into the entire area of the diffuse illumination patch.On the other hand, referring to FIG. 4B, it will be seen that in thiscase each point in the diffuse illumination patch receives light fromthe entire area of the hologram. It will be clear to those skilled inthe art of holographic optics that other optical configurations may beused to form a diffuse illumination patch that exists at any location infront of or behind the holographic diffuser. It should also be apparentthat the same principles may be used to provide a holographic diffuserbased on reflection holograms. Whichever interpretation is used, theeffect is to provide random spatio temporal averaging of the specklepattern, multiple speckle patters are superimposed and the specklecontrast is decreased. If the hologram is now illuminated by light fromthe micro scanner each point on the hologram is illuminated by rayshaving random incident angles covering a small angular sweep. This isequivalent to the classical displaced diffuser approach.

In one embodiment of the invention illustrated in the schematic sideelevation view of FIG. 5 a despeckler comprises a micro scanner 1 basedon the principles described above, a holographic diffuser 2 based on theprinciples described above, The micro scanner is illuminated by lightfrom a laser 21 which is expanded and collimated by the beam couplingoptics 22. The beam coupling optics typically includes a beam expanderand collimator together with means such as a grating or prism foroptically coupling the laser beam to the micro scanner. Specifically,the light from the laser indicated by 101 is expanded and collimated toprovide the beam indicated by 102. The invention does not assume anyparticular optical design for the beam coupling optics. The microscanner deflects the beam 102 in small angular sweeps of randomamplitude indicated by 111, 112 according to the principles discussedabove. The holographic diffuser then diffracts light to form a diffuseillumination patch as discussed above.

FIGS. 6A-6B illustrates the operation of the diffuse illumination patchin more detail. In FIG. 6A illumination from a collimated beam in theray direction 111 resulting in the diffracted ray group 121A-121D whichis uniformly distributed around the average ray direction 121. FIG. 6Billustrates the ray paths follow when the incidence beam being swept tothe ray direction 112 resulting in the diffracted ray group 122A-122Dwhich is uniformly distributed around the average ray direction 122.Each position on the incident rays sweep gives rise to a unique diffuseray distribution across the diffuse illumination patch. Superposing andtemporally integrating the set of such patterns generated by the fullincident ray sweep over the eye integration time results in a reductionof the speckle contrast.

In one embodiment of the invention illustrated in the schematic sideelevation view of FIG. 7 the despeckler of FIG. 5 further comprises abeam steering means for directing the output ray angles 111, 112 fromthe micro-scanner into the input angles 131, 132 required by theholographic diffuser. Typically, the holographic diffuser will requireoff-axis incidence angles. Advantageously, the beam steering means is adiffractive device based on Bragg gratings. However, other means forsteering the beams into the required angles will be apparent to thoseskilled in the art.

In one embodiment of the invention illustrated in the schematic sideelevation view of FIG. 8 a projector incorporating the despeckler ofFIG. 5 further comprises a microdisplay 3 a projection lens 41 and ascreen 42 which is observed from an eye position indicated by the symbol43.

In one embodiment of the invention illustrated in the schematic sideelevation view of FIG. 9 a projector incorporating the despeckler ofFIG. 7 further comprises a microdisplay 3 a projection lens 41 and ascreen 42 which is observed from an eye position indicated by the symbol43.

Advantageously, each point in the holographic diffuser diffracts lightinto the maximum available area of the diffuse illumination patch, asdefined by the active area of a microdisplay, for example. In someembodiments of the invention it may be sufficient for light diffractedfrom any point to fill only a portion of the maximum available area.Easing the diffraction angle range will generally make the hologramprescription less demanding.

In one embodiment of the invention illustrated in the schematic sideelevation view of FIG. 10 two micro scanners 1A, 1B of identicalprescriptions are provided. The micro scanners are stacked and operatedindependently. The number of micro scanners that can be stacked in thisway is limited only by the transmission losses incurred by thesubstrates, HPDLC and ITO. Speckle reduction increases with the numberof layers.

In one embodiment of the invention illustrated in the schematic sideelevation view of FIG. 11 the beam coupling optics comprises a TIRlightguide 24. A coupling grating 23 admits collimated light from thelaser 21 into a TIR path indicated by 103. A second coupling grating 25directs light into the micro scanner. A TIR lightguide may be used tocouple in light from RGB laser sources or multiple monochromatic sourcesdepending on the application.

In one embodiment of the invention the holographic diffuser is a CGH.

In one embodiment of the invention the holographic diffuser is recordedinto a HPDLC using the same procedure as described above. In this casethe holographic diffuser can be switched on and off.

In one embodiment of the invention in which the holographic diffuser isrecorded into a HPDLC the holographic diffuser may be configured as anarray of selectively switchable diffuser elements recorded eachoperating according to the above principles.

In one embodiment of the invention the holographic diffuser also encodesthe properties of beam shaping and homogenization. The principles areknown to those skilled in the art of diffractive optical element design.The holographic diffuser is made by fabricating a CGH with the requiredoptical properties and recording the CGH into the holographic diffuser.(essentially forming a hologram of the CGH).

In one embodiment of the invention illustrated in FIG. 12 there isprovided a despeckler for use with a reflective display. The despeckleris similar to the embodiment of FIG. 7 but further comprise a polarizingbeamsplitter 44 and a quarter wave plate 45. Linearly polarized lightfrom the laser is transmitted through the micro scanner, beam couplingoptics and holographic diffuser as described above to provideillumination light 131,132. The illumination light is transmittedthrough the polarizing beamsplitter transmitted through the quarter waveplate reflected at the microdisplay and transmitted once again throughthe quartet wave plate emerging with polarization orthogonal to that ofthe incident light and is then reflected at the polarizing beam splittertowards projection lens which is not illustrated. The entire reflectedlight path is indicated by 150. In the case of a liquid crystal displaypanel the quarter wave plate will not be required.

The inventors have found that micro scanner according to the principlesdescribed above is polarization sensitive.

In one embodiment of the invention illustrated in FIG. 13 there isprovided a further despeckler for use with a reflective microdisplay.The despeckler is similar to the embodiment of FIG. 5 but furthercomprise a polarizing beamsplitter 44 and a quarter wave plate 45.Linearly polarized light from the laser is transmitted through thepolarization beam splitter, micro scanner, and holographic diffuser asdescribed above to provide illumination light 131, 132. The illuminationlight is transmitted through the quarter wave plate reflected at themicrodisplay and transmitted once again through the quartet wave plate,holographic diffuser and micro scanner and is then reflected at thepolarizing beam splitter towards projection lens which is notillustrated. The entire reflected light path is indicated by 151. Inthis embodiment of the invention the holographic diffuser will ideallybe sensitive to the same polarization as the micro scanner.

In one embodiment of the invention the electrode elements of the microscanner have substantially the same cross sectional area as a prismaticelement.

In one embodiment of the invention the center of said electrode elementof the micro scanner overlaps the vertex of a prismatic element.

In one embodiment of the invention the center of an electrode element ofthe micro scanner is offset from the vertex of a prismatic element.

In one embodiment of the invention the prism array of the micro scanneris a linear array of elements of triangular cross section as illustratedin FIG. 1.

In one embodiment of the invention the prism array of the micro scanneris a two-dimensional array comprising pyramidal elements of crosssection similar to the one illustrated in FIG. 15. In such an embodimentray deflections occur in two directions.

In one embodiment of the invention the prismatic elements of the microscanner are identical. Such an embodiment of the invention is alsoillustrated by FIG. 1.

In one embodiment of the invention the surface angles of the prismaticelements of the micro scanner have a random distribution. Such anembodiment of the invention is also illustrated by FIG. 1.

In one embodiment of the invention the prismatic elements of the microscanner are each characterized by one of at least two different surfacegeometries. Such an embodiment of the invention is also illustrated byFIG. 1.

In one embodiment of the invention the prismatic elements of the microscanner are each characterized by one of at least two different surfacegeometries with the prismatic elements of each surface geometry beingdistributed uniformly across the prism array.

In one embodiment of the invention the prismatic elements of the microscanner have diffusing surfaces.

In one embodiment of the invention the laser source comprises red greenand blue emitters.

The invention is not restricted to any particular laser sourceconfiguration. The HPDLC drive electronics are not illustrated. Theapparatus may further comprise relay optics, beam folding mirrors, lightintegrators, filters, prisms, polarizers and other optical elementscommonly used in displays

The present invention does not assume any particular process forfabricating a despeckler devices. The fabrication steps may be carriedout used standard etching and masking processes. The number of steps maybe further increased depending on the requirements of the fabricationplant used. For example, further steps may be required for surfacepreparation, cleaning, monitoring, mask alignment and other processoperations that are well known to those skilled in the art but which donot form part of the present invention

It will be clear from the above description of the invention that thedespeckler embodiments disclose here may be applied to the reduction ofspeckle in a wide range of laser displays including front and rearprojection displays, wearable displays, scanned laser beam displays andtransparent displays for use in viewfinders and HUDs.

The invention is not limited to any particular type of HPDLC or recipefor fabricating HPDLC. The HPDLC material currently used by theinventors typically switches at 170 us and restores at 320 us. Theinventors believe that with further optimization the switching times maybe reduced to 140 microseconds.

It should be emphasized that the Figures are exemplary and that thedimensions have been exaggerated. For example thicknesses of the HPDLClayers have been greatly exaggerated.

The HPDLC may be based on any crystal material including nematic andchiral types.

In particular embodiments of the invention any of the HPDLC devicesdiscussed above may be implemented using super twisted nematic (STN)liquid crystal materials. STN offers the benefits of pattern diversityand adoption of simpler process technology by eliminating the need forthe dual ITO patterning process described earlier.

The invention may also be used in other applications such as opticaltelecommunications

Although the invention has been described in relation to what arepresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not limited to thedisclosed arrangements, but rather is intended to cover variousmodifications and equivalent constructions included within the spiritand scope of the invention.

1. An illumination device comprising: a source of light; a microdisplay; a beam coupling optics comprising: a total internal reflection (TIR) lightguide; a coupling grating for admitting collimated light from said source into said lightguide; and a second coupling grating for extracting light out of said light guide; and disposed between said beam coupling optics and a microdisplay: a first transparent substrate having an output surface; a variable refractive index layer having an input surface and an output surface, wherein said input surface of said refractive index layer overlays said output surface of said first transparent substrate; a second transparent substrate having an input surface, wherein said input surface of said second transparent layer overlays said output layer of said variable refractive index layer; and a holographic diffuser, wherein said output surface of said variable refractive index layer and said input layer of said second transparent substrate are each configured as a multiplicity of ray deflecting features to deflect incident light into a temporally varying random distribution of ray directions in a first predefined range of angles having a plurality of angles of deflection such that the plurality of angles of deflection are varied as a voltage applied across the variable refractive index is iteratively varied, and wherein each point in said holographic diffuser diffracts incident light rays of said first predefined range of angles into output light rays having a second predefined range of angles forming a diffuse illumination patch.
 2. The illumination device of claim 1, wherein said diffuse illumination patch is disposed in proximity to the surface of a microdisplay or in proximity to the surface of a projection screen.
 3. The illumination device of claim 1, wherein said holographic diffuser comprises a hologram of a scattering surface.
 4. The illumination device of claim 1, wherein said holographic diffuser comprises multiplicity of holograms of scattering surfaces.
 5. The illumination device of claim 1, wherein said holographic diffuser comprises one or more of a group consisting of a Bragg hologram, a switchable Bragg grating, a computer-generated hologram, a transmission hologram and a reflection hologram.
 6. The illumination device of claim 1, wherein said holographic diffuser encodes properties for performing one or both of varying the intensity distribution of said incident light and shaping the cross-sectional geometry of incident light.
 7. The illumination device of claim 1 wherein said holographic diffuser is an array of selectively switchable holographic elements.
 8. The illumination device of claim 1, further comprising one or more of a group consisting of a beam steering means, a beamsplitter, a beam expander, a polarizer, a light integrator, a light guide, and a projection lens.
 9. The illumination device of claim 1, wherein said light source comprises emitters of at least two different colors.
 10. The illumination device of claim 1, wherein said ray deflecting features have three or more facets for deflecting light.
 11. The illumination device of claim 1, wherein said ray deflecting features have surface angles defined by a random distribution.
 12. The illumination device of claim 1, wherein said ray deflecting features are characterized by at least two different geometrical prescriptions.
 13. The illumination device of claim 12, wherein said geometrical prescriptions are distributed uniformly across said input surface of said second transparent substrate.
 14. The illumination device of claim 1, wherein said ray deflecting features have at least one light diffusing surface.
 15. The illumination device of claim 1, wherein electrodes are applied to each the output surface of the first substrate and the input surface of the second substrate, with at least one said electrode patterned into electrode elements for providing addressable regions of said variable refractive index layer, wherein the average refractive index of any of said addressable regions is proportional to a voltage applied across electrodes overlapping said regions, wherein said first range of angles is determined by refractive index of said substrates that the average refractive index of said regions of said variable refractive index layer, wherein the voltage applied across each of said electrode elements is varied temporally.
 16. The illumination device of claim 1, wherein each ray deflecting feature is configured to output a bundle of light rays that overlaps with at least one other bundle of light rays outputted by another ray deflecting feature.
 17. The illumination device of claim 1, wherein said light source is LED or laser.
 18. The illumination device of claim 1, configured to illuminate a reflective microdisplay.
 19. The illumination device of claim 18, wherein light reflected from said reflective microdisplay propagates through said diffuser and through said variable refractive index layer.
 20. The illumination device of claim 18, wherein a beam splitter cube is disposed along the light path from said source to said reflective microdisplay, wherein light reflected from said reflective microdisplay is reflected by said beam splitter cube into said illumination patch. 